Because it has been established that the enzyme glucosyltransferase (GT) contributes significantly to the cariogenicity of Streptococcus mutans, the multiplicity of this enzyme and the genetic locus (or loci) controlling its synthesis are under investigation. In particular, the proposals that GT is encoded by a plasmid or a lysogenic phage are being tested. Experimental data obtained so far with spontaneous GT-deficient mutants of S. mutans LM-7 do not support either of these proposals. To determine if the mutant phenotype is due to a mutation in the plasmid, plasmids from parent and GT mutant strains are being mapped with restriction endonucleases. The GT multiplicity problem is being analyzed by electrophoresis. It was found that serotype c and e strains only had a high-molecular-weight aggregate that synthesized an insoluble glucan. In contrast, serotype d and g strains showed distinct activities of lower molecular weight that synthesized insoluble and soluble glucans, respectively.